The present invention relates to active optical amplifiers in the form of planar optical waveguides.
Optical amplifiers are an important component in optical networks for distributing optical signals. In recent years, erbium-doped optical fibres have been developed which have the capability of amplifying an optical signal. In order to amplify an optical communications signal propagating in an erbium-doped fibre amplifier, light of a different wavelength is coupled into the fibre from a pumping laser. The pumping laser stimulates electronic transitions which amplify the communications signal as it passes through the erbium-doped optical fibre.
In applications where optical components need to be relatively small and device integration is desirable, it is advantageous to provide an optical amplifier in the form of a planar waveguide integrated on a single substrate. However, there are difficulties in integrating erbium-doped amplifiers. In particular, since the longitudinal dimensions of integrated amplifiers tend to be much smaller than the longitudinal dimensions of erbium-doped fibre amplifiers, it is necessary to increase the gain of the amplifier. Attempts have been made to increase the gain by increasing the percentage of erbium. However, the gain in erbium-doped fibre amplifiers has been found to decrease when the erbium doping concentration exceeds a critical level. For example, in silica-based amplifiers, maximum gain is achieved with an erbium concentration of around 0.01-0.02 atomic %. It is believed that at higher concentrations of erbium, the gain is reduced due to an increase in erbium-erbium interactions. One method of addressing this problem has been to increase the solubility of erbium in silica-based glass by incorporating various glass modifiers into the structure, such as sodium and calcium. However, this approach has had limited success, particularly when the core layer of the waveguide is deposited by sputter deposition.
Sputter deposition involves bombarding a target of source material in a manner which ejects electrons and target atoms from the target and deposits at least some of the ejected target atoms onto a substrate. In configurations where a magnet is positioned beneath the target so as to increase plasma densities closer to the target, the technique is referred to as magnetron sputtering. One of the characteristics of sputter film deposition is that different species of target atoms tend to have different deposition rates due to differences in gas scatter rates and substrate sticking coefficients. Thus, a film deposited from a composite target containing a number of different atomic species (e.g. silica, erbium, sodium and calcium) can have a composition which is different to that of the composite target. There is therefore a need for an optical planar waveguide amplifier which has an improved gain, and for an improved method of fabricating the optical planar waveguide amplifier.
In accordance with a first aspect of the present invention, there is provided a planar optical waveguide amplifier for amplifying optical communications signals when optically pumped by radiation of a pumping wavelength, the amplifier comprising:
an optical buffer layer formed on a substantially planar substrate;
an optically-transmissive metal-oxide-based waveguide core formed on the buffer layer, the core comprising aluminium oxide and a gain medium; and
an optical cladding layer formed over the core.
The aluminium oxide may comprise at least 50 mol % of the core. Preferably, aluminium oxide comprises at least 70 mol % of the core. Preferably, the composition of the core predominantly comprises aluminium oxide. In one embodiment, aluminium oxide comprises at least 80 mol % of the core.
The gain medium may comprise one or more lanthanide species such as erbium or ytterbium. In one embodiment, the gain medium comprises both erbium and ytterbium. The inventors have recognised that the solubility of erbium in aluminium oxide is greater than in silica-based materials. Thus, where the gain medium comprises erbium, the present invention has the advantage of enabling higher core concentrations of erbium to be attained than is possible with silica-based cores. The concentration of erbium in the core may be at least 0.1 atomic %, up to a maximum of about 1.0 atomic %. By comparison, known silica-based amplifying cores are doped with a maximum of about 0.01-0.02 atomic % of erbium. In one embodiment, the concentration of erbium in the core is between 0.05 and 0.5 atomic %.
The core may optionally comprise a gain-broadening dopant for broadening a gain spectrum of the amplifier. For example, the gain-broadening dopant may comprise, but is not limited to, fluorine, tellurium, sodium or calcium. The core may optionally comprise at least one refractive-index-modifying dopant, such as fluorine. The core may further comprise a dopant capable of reducing interactions between atoms of the gain medium which decrease the potential gain of the amplifier. For example, where the gain medium comprises erbium, the core may be doped with fluorine to reduce erbium-erbium interactions and to decrease the refractive index of the core.
Preferably, the buffer layer and cladding layer each have a refractive index which is lower than a refractive index of the core, so as to optically isolate the core. The cladding layer and buffer layer may each comprise silica-based materials. The cladding layer and/or buffer layer may comprise a core of another planar waveguide. For example, the amplifier core may be disposed within a silica-based core of another waveguide in order to couple optical signals between the two waveguides.
The core may comprise a material which has been deposited by sputtering, preferably by dc sputtering. The core may have been annealed so as to reduce the amount of defects or impurities which could potentially cause absorption at a wavelength of the communications signal, or non-radiative energy transfer from the gain medium.
The substrate may comprise a wafer of silicon and may incorporate one or more over-layers of material formed upon it. For example, another waveguide may be interposed between the silicon wafer and the amplifier.
In accordance with a second aspect of the present invention, there is provided a method of fabricating a planar optical waveguide amplifier for amplifying optical communications signals when optically pumped by radiation of a pumping wavelength, the method comprising:
forming an optical buffer layer on a substantially planar substrate;
forming an optically transmissive metal-oxide-based waveguide core on the buffer layer, the core comprising aluminium oxide and a gain medium; and
forming an optical cladding layer over the core.
The step of forming the metal-oxide-based core may comprise depositing a metal-oxide based core layer on the buffer layer and shaping the core layer so as to form the waveguide core by means of lithographically-defined etching. The core may have a channel geometry.
The gain medium may comprise one or more lanthanide species such as erbium and/or ytterbium.
The core layer may be deposited by sputtering. The sputtering may be carried out in a sputtering atmosphere containing a noble gas. Preferably, the core layer is deposited by reactive dc sputtering, and most preferably, by reactive dc magnetron sputtering in a sputtering atmosphere containing oxygen. An advantage of dc sputtering is that it can achieve a higher deposition rate than is possible with rf sputtering, and dc magnetron sputtering achieves an even higher deposition rate than conventional dc sputtering. The sputtering atmosphere may further comprise an additional reactive gas capable of incorporating a refractive-index-modifying dopant, such as fluorine, in the deposited core layer. The additional reactive gas may comprise a fluorine-containing gas suitable for incorporating fluorine into the core layer, and may comprise carbon tetrafluorine (CF4). Where the reactive gas contains fluorine, the refractive index of the deposited core layer may be reduced through a formation of aluminium oxyfluorine. A sputtering target used to deposit the core layer may be fluorinated during the deposition to an extent sufficient to increase a sputtering rate of the target and to incorporate fluorine in the core layer. Alternatively, or in addition, the core layer may be fluorinated directly as it grows. The fluorination may be carried out by cyclically flowing a fluorine-containing gas over the target or core layer and then halting the flow of the fluorine-containing gas in a manner which prevents total fluorination (i.e. conversion to AlF3) of the entire core layer. Thus, the core layer may be in the form of a multilayered structure in which the concentration of fluorine changes from layer to layer, preferably periodically, resulting in an average refractive index which is lower than that of pure aluminium oxide.
The core layer may also be formed by simultaneously dc sputtering two targets of aluminium, only one of which is fluorinated, so as to incorporate material sputtered from both targets into the core layer. Preferably, the substrate is exposed to only one of the two targets at a time, for example by cyclically moving the substrate from one target to the other using a substrate rotation stage. Alternatively, the core layer may be formed by simultaneously exposing the substrate to both targets. Preferably, the core layer is deposited by reactively dc sputtering at least one metallic target containing aluminium in a sputtering atmosphere containing oxygen. The ratio of aluminium to gain medium in the core layer can be controlled by controlling the atomic composition of a sputter surface of each target. The gain medium may be disposed throughout the aluminium so as to form an alloy. Alternatively, the gain medium may be concentrated in at least one local region with an aluminium target. For example, the target can comprise an aluminium plate holding at least one plug of gain medium. In a further alternative, the target may comprise a laminate structure consisting of a layer of gain medium, such as erbium or ytterbium, disposed directly behind and in contact with a layer of aluminium, wherein the layer of aluminium includes openings through which the layer of gain medium is exposed during the deposition. The ratio of sputtered aluminium to sputtered erbium can be controlled by controlling the size and number of openings in the aluminium target layer.
The reactive dc sputtering is preferably carried out such that there is a level of ion bombardment at the target surface which is sufficient to prevent surface passivation (i.e. oxide formation) of the gain medium in each target. Where the sputtering atmosphere comprises oxygen mixed with a noble gas, such as argon, the gas flows of oxygen and the noble gas are preferably arranged so as to reduce or prevent oxidation of the exposed target surface. In one embodiment, a noble-gas outlet is provided adjacent the target surface and an oxygen outlet is provided adjacent the substrate surface, so as to produce a higher partial pressure of oxygen at the substrate than at the target surface.
The buffer and cladding layers may comprise silica-based layers and at least one of the buffer layer and cladding layer may be deposited by plasma-enhanced chemical vapour deposition (PECVD). Preferably, each silica-based layer is deposited in the absence of nitrogen or nitrogen-containing gases. Silica-based material which is deposited in this way has the advantage of exhibiting reduced optical absorption in the wavelength range from 1.50 xcexcm to 1.55 xcexcm due to the absence of a nitrogen-induced absorption peak in this region. A liquid source of precursor may be used in the plasma-enhanced chemical vapour deposition to form each silica-based layer. The liquid source of precursor may comprise tetra ethyl oxysilane.
The method may further comprise annealing the core layer and/or core in a manner which reduces, and preferably substantially eliminates, defects and impurities which could potentially cause absorption at a wavelength of the optical signal. The core layer and/or core may be annealed in a manner which reduces, and preferably substantially eliminates, defects and impurities which could potentially cause non-radiative energy transfer from excited atoms of the gain medium.
In accordance with a third aspect of the invention, there is provided a planar optical waveguide amplifier for amplifying optical communications signals when optically pumped with radiation of pumping wavelength, the amplifier comprising:
an optical buffer layer;
an optically-transmissive waveguide core formed on the buffer layer, the core comprising a film of material deposited by reactive dc sputtering; and,
an optical cladding layer formed over the core.
The core may comprise an optically-transmissive metal oxide such as aluminium oxide or titanium dioxide. Alternatively, the core may comprise an optically-transmissive chalcogenide compound, such as zinc oxide or zinc sulfide.
It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.
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